Mr Trump appeared to be referring to the Nord Stream 2 pipeline that would bring gas from Russian Federation to Germany's north-eastern Baltic coast, bypassing Eastern European nations like Poland and Ukraine and doubling the amount of gas Russian Federation can send directly to Germany.

Trump has ramped up his rhetoric ahead of the talks - including in three separate tirades on Twitter on Monday and Tuesday - making the summit one of the most hard in years for the military alliance that has underpinned European security since World War II.

Looking ahead to the wider North Atlantic Treaty Organisation summit agenda, which will include an update on countries' progress on their pledge at a summit in Wales in 2014 to raise defence spending, Mrs May added: "Playing a leading role in North Atlantic Treaty Organisation gives us a powerful voice and we use that to repeatedly make the case for a more modern and adaptable alliance".

"This has been going on for decades and it is disproportionate and not fair to the taxpayers of the United States and we're going to make it fair".

"I have experienced myself how a part of Germany was controlled by the Soviet Union", she said of her youth in Communist East Germany.

"European allies are stepping up", NATO Secretary General Jens Stoltenberg told reporters on Tuesday in Brussels. "NATO has not treated us fairly, but I think we'll work something out".

In a heated meeting in Brussels with the leaders of NATO, Trump claimed that Moscow now basically controls Berlin due to their dependence on it for energy.

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President Trump will be in Europe this week to meet with world leaders - allies and adversaries alike - to discuss pivotal agenda items for American, NATO, and global stability interests.

Previous US presidents also urged greater defence spending from European nations.

Over the weekend, Trump will take a rest stop at one of the two Trump-owned golf courses in Scotland.

But in a bid to appease the U.S. president, they also vow to "submit credible national plans on its implementation". "It is always worth knowing who is your strategic friend and who is your strategic problem", he said.

President Trump stressed he intends to remodel the U.S.

"I think that getting along with Russian Federation, getting along with China, getting along with others is a good thing, not a bad thing", he added. "Frankly, Putin may be the easiest of them all", said Trump. "We'll see how that goes".

But Germany, Europe's biggest economy, spends just 1.24 percent of GDP on defence, compared with 3.5 percent for the US.

And he said: "Dear America, appreciate your allies - after all you don't have that many". At a time when Russian Federation is an worldwide pariah because of its invasions of neighbors like Georgia and Ukraine, the president of the United States has repeatedly denigrated the country's historical allies in Europe while expressing hope that relations with Russian Federation will become closer.

The 1.5 percent level forecast for NATO's European members in 2018 compares to 1.46 percent a year ago, 1.44 percent in 2016 and 1.42 percent in 2015.